Character Clues
Character Analysis
Direct Characterization
The great thing about having Holmes around is that he will generally tell you what new characters are like, often without having even met them. So, we know that Doctor Mortimer is "amiable, unambitious, absent-minded" (1.29) before he's even set foot in Holmes and Watson's apartment—just based on Holmes' looking at his walking stick.
Physical Appearances
Watson loves describing how things look, and he often makes conclusions about people based on their appearances. This makes total sense, since Watson is a limited first-person narrator. After all, Watson can't go into the minds of these other characters to tell us what they're really like. The best he can do is read them thoroughly from the outside to give us a sense of what they are like inside.
For example, when Watson sees Laura Lyons for the first time, he comments on her great beauty, which is slightly ruined by something "subtly wrong" with her face. This "coarseness" (11.4) he sees in Laura Lyons' face tips us off that there is probably also something wrong with her moral character.
Facial appearance is also used to make inferences about the character of Sir Henry, with his "sensitive nostrils" (6.26) and Selden, with his "terrible, animal face" (9.126). It was a popular belief in Victorian England that a person's character was reflected in his face. Fine, attractive features suggested refinement and goodness; coarse features suggested a sinister character. (Shmoop has just two words about this idea: Dexter Morgan. Cute, right?)
Anyway, you'll find this assumption in a lot of Victorian novels.
Social Status
Social status affects characterization in surprising ways in this novel. Being a gentleman by birth is definitely no guarantee of good behavior in The Hound of the Baskervilles: consider Mr. Frankland, the lawsuit-loving, daughter-abandoning crank of Lafter Hall, or, of course, Stapleton, the murdering fraud of Merripit House. In fact, a tendency to criminal behavior seems to be an inherited trait in the Baskerville family, Sir Henry being the exception to the rule. However, it's true that the working-class Barrymores speak differently than Sir Henry Baskerville, and that he questions their loyalty in a way that he might not if they weren't his servants.
Also, you'll notice that all the major characters in this book are gentlemen or ladies (at least, by birth—Laura Lyons works for her living now, thanks to her disastrous marriage). That's because there's a subtle assumption of class division in this book, in which social connections only exist among people of the same class. It certainly doesn't seem to occur to anyone that the blacksmith or the innkeeper in Coombe Tracey might have a reason to murder Sir Charles or Sir Henry—when Dr. Mortimer lists off people in the neighborhood who might be of interest to Holmes, he only mentions the upper-class ones. He wouldn't have been all that familiar with the others.